The present invention relates to a silica glass optical material and a method for producing the same; in particular, it relates to a silica glass optical material for radiation with a wavelength of 155 to 195 nm using an excimer laser or an excimer lamp as the light source, and to a method for producing the silica glass optical material.
The silica glass optical material described above is used as lenses, prisms, windows, reflectors, photomasks, tubes, etc., that are embedded in a light irradiating device for photo-cleaning, aligners for producing integrated circuits (photolithographic devices), etc., using an excimer laser device or an excimer lamp which emits light with a wavelength of from 155 to 195 nm.
Conventionally, for producing patterns of integrated circuits on a silicon wafer, ultraviolet radiation using mercury vapor lamp, such as g-line and i-line, has been used as the light source for a photolithographic device. However, as the semiconductor devices become finer, the aforementioned g-line and i-line found limits in resolution. Accordingly, excimer lasers which emit radiation with shorter wavelength attracted attention, and a photolithographic device using KrF excimer laser (248 nm) has been developed and put into practice.
However, a higher degree of integration of the semiconductor devices is expected in the near future, and this requires a light source capable of producing fine patterns with a line width of 0.1 xcexcm or still finer.
As the light sources meeting the requirements above, there can be mentioned high power output vacuum ultraviolet radiators emitting a wavelength of from 155 to 195 nm. Thus, efforts are devoted mainly to the development of an ArF excimer laser (193 nm), and next to that of an ArCl excimer laser (175 nm), an F2 excimer laser (157 nm), etc. However, because the high power output vacuum ultraviolet radiation is far higher in power compared to the ultraviolet radiation used conventionally in photolithographic devices, the optical materials subjected to the irradiation may suffer abrupt damage such as a drop in transmittance, an increase in refractive index, a generation of strain, a generation of fluorescence, occasional generation of micro-cracks, etc., thereby making the material practically unfeasible.
In addition to the above, dry cleaning using a high power output ultraviolet radiation with a wavelength of from 155 to 195 nm, such as an ArF excimer laser (193 nm), an F2 excimer laser (157 nm), an Xe2 excimer lamp (172 nm), an ArCl excimer lamp (175 nm), etc., is being developed as a method for cleaning semiconductor devices. However, devices for the cleaning treatment require a large optical material for use as the windows and tubes. However, if the optical material becomes larger in size, it tends to suffer greater damage by the high output vacuum ultraviolet radiation, and it no longer serves as an optical material.
In the light of such circumstances, the development of an optical material that suffers less damage by the irradiation with the aforementioned high power output vacuum ultraviolet radiation emitted by an excimer laser or an excimer lamp has been keenly demanded.
As a material that satisfies the aforementioned requirements, there is known a material disclosed in Japanese Patent Laid-Open No. 227827/1994. More specifically, the optical material disclosed in the publication above is a transparent quartz glass produced by heating a porous quartz glass body formed by depositing fine quartz glass particles obtained by flame hydrolysis and growing it, characterized in that the transparent quartz glass contains 10 ppm or less of OH, 400 ppm or more of a halogen, and that it contains hydrogen.
In Japanese Patent Publication No. 48734/1994 an optical material for laser radiation is proposed, having a gaseous hydrogen concentration of at least 5xc3x971016 (molecules/cm3) or higher and an OH group concentration of 100 wtppm or higher. Further, in Japanese Patent Publication No. 27013/1994 a synthetic silica glass optical body is proposed having a hydrogen gas concentration of at least 5xc3x971016 (molecules/cm3) or higher, an OH group concentration of 50 wtppm or higher, and substantially free from distribution in fluctuation of refractive index by canceling out the distribution in fluctuation of refractive index based on the concentration distribution of OH groups by the distribution in fluctuation of refractive index based on the virtual temperature.
However, if the optical material is used in a large optical device, for instance, in an optical device exceeding a size of 200 mm in diameter and 30 mm in thickness, non-uniform distribution likely occurs in the concentration of hydrogen molecules, OH Ad groups, and halogen, and this leads to inferior optical characteristics ascribed to the change in transmittance and refractive index under irradiation with excimer laser or excimer lamp. If OH groups should be contained in the silica glass optical material in such a high concentration as 100 wtppm or higher, the durability becomes inferior due to a drop in the initial transmittance in the vacuum ultraviolet region. That is, the optical material proposed in the above patent publication suffered problems of low initial transmittance in the wavelength region of from 1 55 to 195 nm and of insufficient durability. The optical material disclosed in Japanese Patent Laid-Open No. 227827/1994 utilizes halogens, however, among the halogens, Cl and the like are apt to generate defects upon irradiation of an ultraviolet radiation, and it suffered a serious problem of deteriorating the performance of the optical material such as transmittance in the targeted spectral region.
In the light of such circumstances, it has been found that there can be obtained a synthetic silica glass optical material excellent in durability to long irradiation with an excimer laser or excimer lamp and having high transmittance and a small fluctuation in refractive index, xcex94n, by increasing the purity of the optical material higher than that disclosed in the published patent application above while controlling the concentration of the OH groups and the hydrogen molecules in a certain range, and by making the concentration distribution thereof uniform while particularly selecting Fluorine among the halogens and controlling the concentration thereof to a specified range smaller than that employed in a conventional technology. Furthermore, by limiting the concentration of the OH groups and the hydrogen molecules within the synthetic silica glass optical material in a range narrower than that described above, it has been found that the material can yield a higher initial transmittance, particularly, with respect to an excimer laser radiation in the wavelength region of from 155 to 195 nm, and that the durability can be maintained at a high level. The present invention has been accomplished based on these findings.
That is, an object of the present invention is to provide a silica glass optical material having a high initial transmittance with respect to excimer lasers and excimer lamps emitting radiation in a wavelength region of from 155 to 195 nm and a small fluctuation in refractive index, yet having excellent durability when subjected to an irradiation for a long duration of time.
The problems above can be solved by one of the embodiments described in (1) to (13) below.
(1) A silica glass optical material for transmitting light with a wavelength of from 155 to 195 nm emitted from an excimer laser or an excimer lamp, wherein said silica glass optical material is of ultrahigh purity, contains from 1 to 1 00 wtppm of OH groups, from 5xc3x971016 to 5xc3x971019 molecules/cm3 of H2, and from 10 to 10,000 wtppm of F, but is substantially free from halogens other than F, and has a fluctuation in refractive index, xcex94n, of from 3xc3x9710xe2x88x926 to 3xc3x9710xe2x88x927.
(2) A silica glass optical material of (1) above, wherein the fluctuation in OH-group concentration, xcex94OH, is 30 wtppm or less.
(3) A silica glass optical material of (1) or (2) above, wherein the fluctuation in H2 concentration, xcex94H2, is 3xc3x971017 molecules/cm3 or less.
(4) A silica glass optical material described in one of (1) to (3), wherein the material contains from 12 to 100 wtppm of OH groups and from 3xc3x971017 to 1xc3x971019 molecules/cm3 of H2.
(5) A silica glass optical material described in any of (1) to (4), wherein the material contains from 10 to 380 wtppm of F.
(6) A silica glass optical material described in any of (1) to (5), wherein the material is of ultrahigh purity containing, as impurities, 5 wtppb or less of each of Li, Na, and K, 1 wtppb or less of each of Ca and Mg, and 0.1 wtppb of each of Cr, Fe, Ni, Mo, and W.
(7) A silica glass optical material described in any of (1) to (6), wherein the concentration of oxygen-deficient type defects which generate an absorption band at 7.6 eV is 1xc3x971017 defects/cm3 or less.
(8) A silica glass optical material described in any of (1) to (7), wherein the material contains 10 wtppm or less of Cl.
(9) A silica glass optical material described in any of (1) to (8), wherein the material is used for an optical device having an optical path length from an excimer laser or an excimer lamp of 30 mm or longer.
(10) A silica glass optical material for transmitting light with a wavelength of from 155 to 195 nm emitted from an excimer laser or an excimer lamp, characterized in that said silica glass optical material is of ultrahigh purity, contains from 1 to 100 wtppm of OH groups, from 5xc3x971016 to 5xc3x971019 molecules/cm3 of H2, and from 10 to 10,000 wtppm of F, but is substantially free from halogens other than F, and satisfies a relation that the sum of a and b is 100 wtppm or higher and that the ratio b/a is in the range of from 1 to 1000, wherein a and b each represent the content of OH groups and that of F, respectively.
(11) A silica glass optical material of (10), wherein the ratio b/a is in the range of from 10 to 100.
(12) A method for producing a silica glass optical material, which comprises producing a white colored soot body containing OH groups by means of flame hydrolysis of a silicon compound, performing fluorine doping treatment to the resulting soot body by heat treatment in a fluorine-containing gaseous atmosphere to obtain a white soot body containing OH groups and fluorine, vitrifying the resulting body to obtain a transparent body, obtaining a rod-like transparent silica glass body by flame heat molding, applying to the resulting body zone melting rotary stirring treatment by flame heating to thereby homogenize the distribution of concentration of OH groups and fluorine, removing strain by annealing treatment, and finally performing gaseous hydrogen doping by applying heat treatment in a gaseous atmosphere containing hydrogen molecules.
(13) A method for producing a silica glass optical material of (12), wherein the annealing treatment is performed in a gaseous atmosphere containing hydrogen molecules, thereby performing simultaneously the annealing treatment and the hydrogen gas doping treatment.
In the present invention, further improvements in resistance against excimer laser radiation and in resistance against excimer lamp radiation, as well as in precision in processing using excimer laser or excimer lamp, were achieved by optimizing the combination of the five characteristics of the material, i.e., ultra-high purity, content of OH groups, content of fluorine (F), dissolved gaseous hydrogen, and the fluctuation of refractive index, xcex94n.
The reasons for the necessity of controlling the combination of the five characteristics above are as follows.
Concerning ultra-high purity, increased transmittance and reduced energy absorption in the vacuum ultraviolet region can be achieved by reducing the concentration of metallic impurities in the silica glass. It is required that the concentrations of Li, Na, and K are each 5 wtppb or lower, that those of Ca and Mg are each 1 wtppb or lower, and that those of Cr, Fe, Ni, Mo, and W are each 0.1 wtppb or lower. Li, Na, K, Ca, and Mg are contained as impurities of various types of heat-resistant ceramics, and they tend to function as contaminating elements in case of producing silica glass. Cr, Fe, Ni, Mo, and W are the components of the structural material used in plants. In particular, Mo and W are used as heat-resistant metals, and they also tend to function as contaminants.
The OH groups are the terminal ends of the network structure of glass; by adding them in a proper amount, the structure can be relaxed and the Sixe2x80x94Oxe2x80x94Si bonding angle can be brought close to a stable value. However, if OH groups should be incorporated in a high concentration, they lead to a drop in transmittance in the vacuum ultraviolet region. Thus, Oh groups should be incorporated in a concentration range of from 1 to 100 wtppm, and in particular, in case of a material for excimer lasers irradiating radiation from 155 to 195 nm in wavelength, which is used under severe conditions of high irradiation energy density per unit area, the concentration thereof is preferably in the range of from 12 to 100 wtppm.
Similar to OH groups, F also forms the terminal ends of the network structure of lass. Furthermore, unlike other halogen elements, F, although incorporated at a high concentration, does not cause deterioration in transmittance in the vacuum ultraviolet region. However, if F alone is incorporated at a high concentration in the absence of OH groups, glass undergoes decomposition during the heat treatment, and generates gaseous F2 or absorption band at 7.6 eV (ca. 165 nm) ascribed to the generation of oxygen deficient defects. Accordingly, the key is to incorporate F and OH groups at the same time to suppress the thermal decomposition of glass and the generation of oxygen deficient defects.
From this point of view, it is preferred that the values a and b, where a represents the content of OH groups and b represents the content of F, satisfy the relation as such that a and b in total is 100 wtppm or higher. and that the ratio b/a is in the range of from 1 to 1000. In particular, the ratio b/a preferably is in the range of from 10 to 100. In this case, it is preferred that the concentration of OH groups is in the range of from 1 to 100 wtppm, particularly, 12 to 100 ppm, and that of F is in the range of from 50 to 10,000 wtppm, and particularly, 50 to 380 wtppm.
In the optical material according to the present invention, it is preferred that it does not substantially contain halogens other than F. Because Cl generates a drop in transmittance of glass in the vacuum ultraviolet region (i.e., the wavelength region of excimer laser radiation), it is preferred that the content thereof is 10 wtppm or lower.
A The dissolved gaseous hydrogen, i.e., the hydrogen molecules H2 incorporated inside the optical material, suppresses the generation of an Exe2x80x2 center (denoted as xe2x80x9cE prime centerxe2x80x9d, and yields an absorption band at about 215 nm) or a NBOH center (denoted as xe2x80x9cNon-Bridging Oxygen Hole centerxe2x80x9d, and yields absorption bands at about 260 nm and about 360 nm)(for reference, see S. Yamagata, Mineralogical Journal, Vol. 15, No. 8 (1991), pp. 333-342), and the content thereof is preferably in the range of from 5xc3x971016 to 5xc3x971019 molecules/cm3, and particularly preferably, in the range of from 3xc3x971017 to 1xc3x971019 molecules/cm3.
In case the optical material is used so thin as such corresponding to a thickness of a photomask as disclosed in the above Japanese Patent Laid-Open No. 227827/1994, that is, if the optical path for the laser radiation is so short as about 2 to 3 mm in length, there is no particular problem. However, in case of a product such as a lens used as an optical device having a thickness of 30 mm or more, the precision in processing using the product tends to become inferior if there is a large fluctuation in refractive index, xcex94n. Accordingly, xcex94n is preferably minimized as possible. However, as described above, it has been newly found that, particularly in case F is doped at a high concentration, xcex94n increases due to the distribution in concentration. Hence, in the optical material according to the present invention, the fluctuation in refractive index. xcex94n, was set at a small value in the range of from 3xc3x9710xe2x88x926 to 3xc3x9710xe2x88x927 by applying the treatment described later in the production method.
The fact that xcex94n is set at such a low value signifies that the fluctuation in density of the material is also minimized. As a result, it enables dissolving gaseous hydrogen at a uniform concentration. A value for xcex94n of 3xc3x9710xe2x88x926 or less requires that the material is free of striae in one direction. A glass having a high value of xcex94n contains OH groups and F with a non-uniform distribution in concentration, and the concentration of saturated gaseous hydrogen is presumably influenced by the concentration of such OH groups and F.
From the facts above, the optical material according to the present invention preferably has a fluctuation in concentration of OH groups, xcex94OH, of 30 wtppm or less, and a fluctuation in concentration of F, xcex94F, of 50 wtppm or less. It is also preferred that the fluctuation in concentration of H2, xcex94H2, is 3xc3x971017 molecules/cm3 or less. The concentration of oxygen deficient type defects which generate the 7.6 eV absorption band is preferably not higher than 1xc3x971017 defects/cm3.
The method for producing the silica glass optical material above according to the present invention is described below.
To produce a silica glass optical material according to the present invention, a white soot body containing OH groups is synthesized by means of flame hydrolysis using a silicon compound as the starting material.
As the silicon compound above, there can be used, for instance, SiCl4, SiHCl3, SiH2Cl2, SiCH3Cl3, Si(CH3)2Cl2, SiF4, SiHF3, SiH2F2, etc. As the flame, there can be used an oxyhydrogen flame, a propane oxygen flame, etc.
The white soot body containing OH groups is subjected to fluorine doping treatment by performing heat treatment in a fluorine-containing gaseous atmosphere.
As a gas containing fluorine, preferred is to use a gas containing from 0.1 to 100 vol. % of SiF4, CHF3, SF6, etc. The treatment is preferably performed in the temperature range of from 400 to 1200xc2x0 C., under a pressure of from 0,1 to 10 kgf/cm2 (about 0,01 MPa to 1 MPa; exactly: 1 kgf/cm2=0,0980665 MPa)
The resulting white soot body above is then subjected to vitrification treatment to obtain a transparent body. The treatment is preferably performed in an atmosphere (which may contain He) of reduced pressure of 0.1 kgf/cm2 (about 0,01 MPa) or lower, in the temperature range of from 1400 to 1600xc2x0 C.
Subsequently, the resulting body is molded into a rod-like transparent silica glass body using flame heating, and is subjected to a zone melting rotary stirring treatment. The treatments above can be performed by using the methods disclosed in, for example, U.S. Pat. No. 2,904,713, U.S. Pat. No. 3,128,166, U.S. Pat. No. 3,128,169, U.S. Pat. No. 3,483,613, etc. In particular, as described above, the treatments are thoroughly performed so that a fluctuation in refractive index, xcex94n, should fall in the range of from 3xc3x9710xe2x88x926 to 3xc3x9710xe2x88x927.
To remove strain, annealing treatment is applied to the resulting body. The treatment is generally carried out in the atmospheric air, and also usable are other types of inert gaseous atmospheres. The treatment is performed by holding the body at a temperature range of from 900 to 1200xc2x0 C. for a duration of from 1 to 100 hours, and the temperature is gradually lowered to 500xc2x0 C. or lower at a cooling rate of 1xc2x0 C./hr to 10xc2x0 C./hr.
Finally, doping treatment using gaseous hydrogen is performed by carrying out heat treatment in an atmosphere containing hydrogen molecules. As the atmosphere containing hydrogen molecules, it is preferred to use 100% gaseous hydrogen or a mixed gas atmosphere containing a rare gas such as Ar and gaseous hydrogen. Preferably, treatment is performed at a temperature of from 100 to 800xc2x0 C., and particularly, from 200 to 400xc2x0 C. If the treatment should be performed at a temperature higher than the range specified above, the reductive function becomes too intense as to generate oxygen deficient type defects. On the other hand, if the temperature should be lower than the range above, it takes too long a time for gaseous hydrogen to diffuse and dissolve into the transparent glass body.
Preferably, the treatment pressure is in the range of from about 1 kgf/cm2 to 100 kgf/cm2 (about 0,1 MPa to 10 MPa). Under an atmosphere of 100% gaseous hydrogen at a pressure of 1 kgf/cm2, the saturation dissolution concentration of gaseous hydrogen into transparent glass body is in the range of from about 1xc3x971017 to 4xc3x971017 molecules/cm3; under pressures of 10 kgf/cm2 and 100 kgf/cm2 (about 1 MPa to 10 MPa), the saturation dissolution concentration is 1xc3x971018 to 4xc3x971018 and 1xc3x971019 to 4xc3x971019 molecules/cm3, respectively.
The material thus obtained is prepared intQ a desired shape by grinding the outer surface.